1. Field of the Invention
The present invention relates to a level meter displaying method and a level meter apparatus which can be suitably applied to a VU (Volume Unit) meter for use in audio equipment.
2. Description of the Related Art
VU (Volume Unit) meters used in audio equipment are capable of displaying the average level of a sound signal in the range from a low level of not greater than −20 dB to such an excessive input level of +3 dB (refer to FIG. 1A). VU meters and other ordinary indicator type analog voltmeters are configured such that the input voltage and the rotational angle of an indicator are proportional to each other. Therefore, if the manufacturer of a VU meter defines two reference points consisting of a decibel value at an indicator angle of 90° and an indicator angle at −20 dB, indicator angles for other decibel values are uniquely determined. This will now be concretely described with reference to a characteristic diagram of FIG. 1B showing a conversion line which represents the relationship between indicator angles and ratios of input signal voltages to the reference level. In FIG. 1B, the abscissa indicates the voltage ratio of the input signal voltage to the reference level, and is graduated at regular intervals from 0 times to √2 times (3 dB) of the reference level. On the other hand, the ordinate indicates the indicator angle, and is graduated from 0 to 150 degrees. For example, if the manufacturer of the VU meter sets an indicator angle of 138° for −20 dB, i.e. a voltage ratio of 0.1, and sets an indicator angle of 90° for −3 dB, an indicator angle of 34° is uniquely determined for +3 dB, i.e. a voltage ratio of √2.
Here, it is assumed that a rhythm tone signal is input to the VU meter configured as described above. FIG. 2A is a waveform chart showing a rhythm tone signal which is normalized such that the possible maximum value of positive and negative vibrations lies inside a range of ±100%, and is input to the VU meter. The peak of the signal is at a full scale (100%) of 0 dB, but the average absolute value of signal levels is as low as around −20 dB.
This raises the problem that the indicator of the VU meter constantly indicates values around −20 dB in FIG. 1A, and the position of the indicator is hardly changed from the left end.
Therefore, to increase the rate of change in the position of the indicator at such a low level as −20 dB, it can be envisaged that an indicator angle of 90° is set for −6 dB, and an indicator angle of 145° is set for −40 dB as shown in a characteristic diagram of FIG. 2B. This raises the problem that although a signal level of 0 dB is set for an indicator angle 34°, the indicator angle is −12° below a horizontal line corresponding to 0° at such an excessive input level as 3 dB, and hence lies outside a normal voltmeter indication range. Namely, the conventional level meter cannot change the rate of change in indicator angle in a particular input signal level range such as a low level range. It should be noted that FIG. 2C shows the positional relationship between a plurality of indicator angles corresponding to decibel values (−40 dB, −6 dB, 0 dB, and 3 dB) at main points. In FIG. 2C, positive indicator angles are shown counterclockwise in order with the indicator angle corresponding to the horizontal line being assumed to be 0°.